In the manufacture of paper products, an aqueous suspension containing pulp fibers is formed. The aqueous suspension is spread over a forming surface to form a paper web. The forming surface generally includes a series of endless rotating conveyor belts formed from a porous fabric that is comprised of metal, plastic, or other suitable material. Forming fabrics are designed to facilitate the formation of the non-woven web, to transport the non-woven web, and to remove excess liquid from the web as it travels downstream. When forming low basis weight paper products, such as tissues, the non-woven web is formed between a pair of forming fabrics. In these systems, the suspension of fibers is injected between a pair of moving fabrics as the fabrics are wrapped around a roll.
The quality of paper formation indicates the degree of fiber variance in a paper sheet. The measure of the degree of fiber variance takes into account the size and distribution of holes, flock distribution, and dust particle measurements. Generally, in the past this measurement has been accomplished by taking a sheet end product which has been through essentially all the steps of papermaking, and placing that sample sheet on an inspection box with a light source to visually examine transparency distribution of the sheet.
Tissue making comprises many different steps, and each step changes the properties of the tissue sheet. Tissue sheets are commonly processed using techniques such as creping the tissue from a rotating drum, which involves physically scraping the tissue with a blade from a drum to which the tissue is affixed and dried. This creping step greatly softens the “doctor” tissue, and changes dramatically the texture of the tissue. Furthermore, other types of tissues undergo a process of foreshortening and throughdrying known as UCTAD (uncreped through air drying) which includes passing air through the tissue sheet as it rests upon a forming fabric. See, for example, U.S. Pat. Nos. 5,932,068 and 5,772,845 to Farrington et al. and U.S. Pat. No. 6,017,417 to Wendt et al.
One of the difficulties in examining finished tissue products to determine the quality of paper formation and performance of the headbox and forming roll section is that there are so many process steps beyond the headbox (i.e., downstream of tissue formation) that it is difficult to correlate the properties of the final tissue product with the performance of the headbox and slurry handling apparatus at the beginning stages of tissue manufacture. It would therefore be desirable to develop an apparatus and method that can accurately measure the degree of paper formation and fiber variance in a paper sheet while the paper sheet is still wet, i.e., during the process of paper sheet formation. A method and apparatus of determining, in real time, the properties of a wet paper sheet as it is being manufactured on a twin wire forming fabric would be desirable. Further, a method of using information obtained during tissue manufacture to adjust, in real time, the processing variables and parameters which are responsible for the properties of the sheet would be a valuable technique for tissue product developers.